Currently, thinning and lightening of touch display panels for electronic products are increasingly demanded by manufactures and users. Accordingly, researches on integration of touch panel functionalities with a liquid crystal panel are becoming more and more popular as comparing to the conventional method in which a touch panel is simply arranged on a liquid crystal panel. In-cell touching technology is one of the technologies for integrating a touch panel with a liquid crystal panel, where the touch display screen functionalities are embedded into liquid crystal pixels.
In the up-to-date in-cell touching technology, touch sensor electrodes, i.e. touch driving electrodes Tx and touch sensing electrodes Rx, are fabricated on a TFT array substrate of a liquid crystal panel. In comparison with conventional capacitive touch display screen structures, this novel technique would save a touch glass substrate for every single structure, and thus make the entire display module thinner and lighter. FIG. 1 shows a diagram of a circuit structure of a liquid crystal screen using the in-cell touching technologies.
However, since the touch driving electrode Tx and the touch sensing electrode Rx are both fabricated on the TFT array substrate, there could be various parasitic capacitances and coupling capacitances between these electrodes and respective layers of electrodes of TFT. When the display is in operation, alternating variations of the voltages on TFT will introduce crosstalk to signals on the touch driving electrode Tx and the touch sensing electrode Rx, and thus significantly degrades the touching performance of the liquid crystal screen.
Additionally, in the conventional method, signal scanning of the touch driving electrode Tx and the touch sensing electrode Rx is synchronized with screen refreshing, because capacitive touch display screens will not be affected by the TFT driving signals. In contrast, the current in-cell touching technology utilizes a scanning manner that is conducted in a blank interval between two successive frames, during which the voltages output to the TFT by the source driver and gate driver both stop changing, and the crosstalk introduced to the touch driving electrode Tx and the touch sensing electrode Rx by the driving signals is at the lowest level. However, the electric charges on the parasitic and coupling capacitors will introduce crosstalk to the signals because the touch driving electrode Tx line and the Vcom line are shared.
The flow chart for current in-cell touch display screen driving is shown in FIG. 2: when the touch control integrated circuit (IC) is performing initialization scanning, the liquid crystal panel is not driven yet, and the amount of charges on the parasitic capacitor Cs is zero; after writing of data of the last line in each frame is completed, the touch control IC performs scanning and compares data obtained by this scanning with data obtained during the initialization scanning so as to determine if any touch behavior just occurs. As shown in FIGS. 3 and 4, by graphing the comparison result of the touch control IC, we can see that, even if there is no touch, the comparison data may also vary when the picture is changed (as shown in FIGS. 3A-3B and 4A-4B), due to the effect of the charges on the parasitic capacitor Cs (as shown in FIGS. 3C and 4C), and this will lead to erroneous judgment.